A lithographic projection apparatus is used to image a pattern (e.g. in a mask) onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, and a hard bake. These procedures are used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Each procedure or process may be followed by an inspection of the substrate in an inspection apparatus. With the inspection results, one may optimize or improve the procedures prior to the inspection or if a large part of the substrate is faulty, the patterned layer may be stripped off the substrate and the stripped patterned layer may be reapplied through exposure and/or other lithographic processing. Further information regarding lithographic processes used in, for example, the semiconductor industry can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
Inspection apparatus for surface inspection of substrates may measure properties like line width, pitch and critical dimension (CD) of the patterned layer. A common technique used to perform such inspection is known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol.15 no.2 361-368 1997 and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE Vol. 3677, 1999. In scatterometry, white light is reflected by a patterned parts of a sample (e.g., periodic structures) of a substrate and the resulting reflection spectrum at a given angle is detected. The pattern giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled-Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation.
The monochromatic light response from a surface may be described in terms of four experimental properties:    1) The Transverse Magnetic Reflectivity (RTM) which is the zero order reflectivity of light having its polarization parallel to the plane of incidence.    2) The Transverse Electric Reflectivity (RTE) which is the zero order reflectivity of light having its polarization direction perpendicular to the plane of incidence.    3) The phase change (DTM) of the light in the zero order reflectivity having its polarization direction parallel to the plane of incidence.    4) The phase change (DTE) of the light in the zero order reflectivity having its polarization direction perpendicular to the plane of incidence.
With a technique called reflectometry, it is possible to measure the absolute values RTM and RTE. With another technique called ellipsometry, it is possible to measure the ratio of the two reflectivities (RTM/RTE) and the difference between the two phase changes (DTM/DTE). However, not with one or any combination of these techniques may all four of the above-referenced experimental parameters be determined. The reason is that only the difference of the two phases (DTM/DTE) is measured in ellipsometry and reflectivity does not provide any information about the phase. Thus, these techniques do not specifically measure both DTM and DTE.